A large number of applications exist for alarm circuits which employ a detector of some type to detect the condition for which an alarm is to be activated. This detector then is coupled with a switching and driving circuit to turn on an audible or visual alarm whenever the alarm condition is detected by the system. In its simplest form, such alarm circuits merely comprise a mechanical or electrical switch which is closed upon sensing the alarm condition to complete a circuit to a bell, horn or visual alarm in the form of a light or other device. Alarms of this type are commonly used as burglar alarms to detect the unauthorized entry into a premises when the alarm system has been activated, such as when the premises are normally vacated.
Some applications, however, have arisen which require alarm interface circuits between the sensing device which senses the alarm condition and the audible or visual output which indicates that the alarm condition has been sent. Among these applications are smoke detector alarm circuits which employ either an ionization chamber device or a photoelectric device to detect the presence of smoke and therefore activate the alarm condition. The ionization chamber smoke detectors are popular and require a very high impedence load in order to properly operate, because of the inherent characteristics of the smoke detector device itself. In the past, discrete transistor switching circuits have been used as interface switching amplifiers between the smoke detector and the audible or visual alarm to be driven upon detection of the smoke by the detector.
The use of discrete circuit components, however, is relatively costly and requires more labor than if integrated circuit technology were used. It has been difficult to obtain the desired low current leakage on the output lead from the ionization chamber of the smoke detector, since conventional integrated circuit technology does not provide the necessary high impedance input which is necessary to successful operation of such a circuit.
It is desirable, however, to provide an integrated driver circuit capable of interfacing between a smoke detector, of either the ionization type or photoelectric type, and an output horn, either of the mechanical type of a piezoelectric horn, without any buffer stages between the integrated circuit and either its input from the smoke detector or its output to the horn which it is driving.
In addition, it is desirable to provide, on the integrated circuit itself, an auxillary detector for activating an alarm whenever the battery supply voltage used to operate the circuit falls below some pre-established level. This is necessary since smoke detector circuits must use a DC battery, at least as a backup device, in the event that the electricity is cut off at the time the smoke detector must function. As is well known, DC batteries tend to drop in their voltage output with age; so that from time to time such batteries need to be replaced in most installations.
For some applications of smoke detectors, it also is desirable to be able to interconnect a number of different smoke detectors to a common alarm bus; so that the direction of smoke or an alarm condition by any one of the detectors will activate all of them. Such an installation, for example, is highly desirable in an apartment building where a separate smoke detector and alarm might be placed in each different apartment; but an alarm should be sounded in all of the apartments in the event any one of them senses an alarm condition so that the entire building could be evacuated in time to prevent serious injury or death.